1. Technical Field
The invention relates generally to offshore systems employed for conducting petroleum-related operations, such as drilling and testing productivity of a well, producing fluids from a reservoir, and so forth. More specifically, the invention relates to an apparatus for connecting retrievable subsea components of the offshore system.
2. Background Art
Offshore systems that are adapted for conducting petroleum-related operations in relatively deep water generally comprise a floating vessel, a marine riser, a subsea wellhead, and a subsea blowout preventer stack. The wellhead is positioned below the floating vessel and secured to the seafloor. The blowout preventer stack is mounted on the wellhead and connected to the floating vessel by the marine riser. The marine riser provides a conduit through which tools and fluid can be communicated between the floating vessel and one or more wells beneath the wellhead. Typically, a dynamic positioning system which comprises active means of monitoring position combined with thruster control is used to keep the floating vessel on station. However, a dynamically positioned vessel is subject to drive-off, i.e., rapid evacuation from the operation site, at all times. A drive-off situation may be caused by a number of reasons, some of which include problems with the active means of monitoring position, failure of thrusters, power shutdown on the vessel, storm, and ocean current anomalies.
In a drive-off situation, the marine riser must be disconnected from the blowout preventer stack to permit the vessel to evacuate the operation site. However, before disconnecting the marine riser, the well must be controlled and prepared for abandonment. In some offshore systems, subsea intervention trees, also called subsea internal trees, provide the vessel with the ability to control and quickly disconnect from the well. The subsea intervention tree is usually secured in the blowout preventer stack and includes a valve assembly and a latch assembly. The valve assembly includes one or more valves which may be operated to control and seal the well. The latch assembly includes a lower mating portion and an upper mating portion. The lower mating portion is attached to the valve assembly and the upper mating portion is coupled to a landing string. When the mating portions are connected, the subsea intervention tree can be lowered into the blowout preventer stack on the landing string. The upper mating portion can be released from the lower mating portion to allow the landing string to be retrieved from the blowout preventer stack and pulled to a height which will permit the vessel to leave the operation site safely.
After the emergency event, the vessel may return to the operation site and again re-connect to the well. The landing string with the upper mating portion can be lowered into the blowout preventer stack to allow the upper mating portion to re-connect to the lower mating portion. The upper mating portion typically includes hydraulic connectors which are arranged in a certain order and which must be properly connected to similarly arranged hydraulic connectors on the lower mating portion. To allow proper connection of the connectors, alignment devices are generally provided on the mating portions. These alignment devices will not allow the upper mating portion to contact the lower mating portion until the upper mating portion achieves a certain positional alignment with respect to the lower mating portion. The landing string is usually rotated to place the upper mating position in the desired positional alignment with respect to the lower mating portion. However, in deep water, the weight and length of the landing string make it difficult to properly align the mating portions by rotating the landing string. Therefore, it is desirable to have a latch mechanism with mating portions that can re-connect without the need to manipulate the landing string to achieve a certain positional alignment between the mating portions.